Physicists Make the First Images With Laser-Generated Muons, Seeing Through Concrete and Lead
A Romanian team used a high-power laser to build an artificial muon beam and captured the shadow of a pile of lead bricks behind a 2-meter concrete wall.
Physicists in Romania have produced the first images made with an artificial beam of muons created by a laser, a step that could make a powerful way of seeing inside dense objects far faster. The work is a preprint posted to arXiv and has not yet been through peer review.
Muons are particles similar to electrons but about 200 times heavier. They form naturally when cosmic rays hit the upper atmosphere, and because they lose little energy in matter, they can pass through many meters of rock or metal. By counting how many come out the other side, and how their paths bend, researchers can map the interior of an object. The technique is called muography, and it revealed a hidden chamber inside the Great Pyramid of Giza in 2023.
The weakness is speed. Only about one cosmic muon crosses each square centimeter every second, so a scan can take months. An artificial source could change that, but it has been hard to make muons energetic enough for imaging and to separate them from the other particles produced alongside them.
A team led by Madalina Dobre at the Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering used the ultra-powerful laser at the Extreme Light Infrastructure Nuclear Physics facility, ELI-NP, in Romania. The laser fired into a gas and accelerated electrons to extremely high energy. When the electrons hit a solid target they produced bursts of light that then created pairs of muons.
To remove the unwanted particles, the beam went through a large filter of plastic and paraffin and then a concrete wall 2 meters thick. Portable detectors in a van, as far as 42 meters from the source, recorded the shadow cast by a pile of lead bricks. The shadow closely matched the known size and position of the bricks. Comparison with detailed simulations indicated that about 90% of the detected particles were muons with energies consistent with a laser-driven source.
That makes the result the first image dominated by laser-driven muons. The resolution is still low, and better detectors and lasers will be needed before the method can handle complex objects.
The team says on-demand muon beams could eventually be used to inspect aging infrastructure or to let border officials look for nuclear material hidden inside heavy shielding. Both jobs are hard with X-rays, which do not penetrate thick metal, and slow with cosmic muons.
The paper is titled "Imaging with GeV muons produced via laser-wakefield-accelerated electrons." Independent groups will need to reproduce the result, and the peer-review process will test the claim that nine in ten detected particles were muons, but the demonstration shows the basic idea works.
